These notes provide a comprehensive review of the semiclassical approach for calculating multiparticle production rates for initial states with few particles at very high energies. In this work we concentrate on a scalar field theory with a mass gap. Specifically, we look at a weakly-coupled theory in the high-energy limit, where the number of particles in the final state scales with energy, , and the coupling with held fixed. In this regime, the semiclassical approach allows us to calculate multiparticle rates non-perturbatively.
Khoze, V. V., & Reiness, J. (2019). Review of the semiclassical formalism for multiparticle production at high energies. Physics Reports, 822, 1-52. https://doi.org/10.1016/j.physrep.2019.06.004